This invention relates to a noise cancel circuit capable of cancelling noise components from a pulse signal without a change of a duty ratio. Such a noise cancel circuit is particularly useful in an electronic device such as a television receiver, a telephone set, and so on.
A pulse signal is used in an electronic device such as a television receiver and a telephone set. In such an electronic device, the pulse signal generally comprises a sequence of pulses each of which has leading and trailing edges. Each of the pulses is defined by a predetermined pulse period, a predetermined pulse width, and a predetermined duty ratio defined by the predetermined pulse period and the predetermined pulse width. There is a case that the pulse signal has noise components while the pulse signal passes through a variety of circuit elements in the electronic device. As an example of the noise components, leading and trailing noises are well known. The leading and the trailing noises originate from the leading and the trailing edges, respectively, of the pulses and have a noise width. In other words, the leading and the trailing noises occur at the leading and the trailing edges. At any rate, such the leading and the trailing noises must be cancelled from the pulse signal.
For this purpose, a noise cancel circuit is used in the electronic device. In this case, the pulse signal is supplied to the noise cancel circuit as an input pulse signal. The noise cancel circuit comprises a delay circuit and a D-type flip-flop circuit. The delay circuit is for giving a delay time duration to the input pulse signal and produces a delayed input pulse signal having the delay time duration. The delay time duration is longer than the noise width and is shorter than the predetermined pulse width. The D-type flip-flop circuit has a D-input terminal and a clock terminal. The D-input terminal is directly supplied with the input pulse signal while the clock terminal is supplied with the delayed input pulse signal. As a result, the D-type flip-flop circuit produces an output pulse signal in which the leading noise is cancelled. Such a noise cancel circuit is disclosed in Japanese Unexamined Patent Publication No. 98014/1986 (Tokkai Sho 61-98014).
In this case, the output pulse signal comprises a sequence of output pulses each of which has an output pulse width. The output pulse width is equal to a difference between the predetermined pulse width and the delay time duration. This means that the duty ratio of the output pulse signal is different from that of the input pulse signal. Furthermore, the noise cancel circuit mentioned above can not cancel the trailing noise.